In recent years, due to rapid technical innovations of ink-jet recording, the resulting print quality is approaching that of silver halide photography. Print quality achieved by ink-jet recording depends mainly on three elements consisting of a printer, an ink and a recording sheet. From the viewpoint of the image quality, the former two elements are subjected to recent marked technical innovations. Accordingly, difference in performance of ink-jet recording sheets (hereinafter occasionally referred to simply as recording sheets) has been considered to critically affect final print quality.
In order to prepare print quality approaching that of silver halide photography, utilizing ink-jet printing, recording sheets have been improved in various aspects. Specifically, recording sheets, which comprise a reflective support prepared by covering both sides of paper as a support with polyolefin resins such as polyethylene, having thereon an ink absorbing layer, have been widely employed to produce prints due to the following reasons. The cost is relatively low compared to plastic film and high grade quality prints approaching those of silver halide photography are produced which exhibit a feeling of depth, flexibility, smoothness, and glossiness.
Ink-jet recording sheets are divided into two main types, that is, one which is ink-absorptive like paper and the other which comprises a support having thereon an ink absorbing layer. Incidentally, the former has not been capable of producing high grade prints due to the fact that since ink penetrates directly into the support, it is difficult to achieve high maximum density and since the support itself absorbs ink solvents, marked image-shaped wrinkling results. Further, known are a number of types of ink-jet recording sheets which comprise a support having thereon an ink absorbing layer. However, problems occur in which when the support absorbs ink solvents, wrinkling tends to occur, and during storage, density tends to decrease due to gradual penetration of dyes adhered into the ink absorbing layer.
Contrary to this, ink-jet recordings sheets, which comprise a so-called non-water absorptive support which does not absorb ink at all, having thereon an ink absorbing layer, exhibit advantages such that aforesaid problems do not occur and it is possible to produce ink-jet prints with a feel of high grade.
Known as ink absorbing layers provided on a support, are two main types, a swelling type ink absorbing layer and a void type or a porous ink absorbing layer.
The swelling type ink absorbing layer is comprised mainly of hydrophilic polymers such as gelatin, polyvinyl alcohol, polyvinylpyrrolidone, and polyethylene oxide.
The void type (or porous) ink absorbing layer is prepared employing several production methods. The representative layer comprises a small amount of hydrophilic polymers and a large amount of minute particles. Voids are formed among the minute particles, into which water is absorbed. This type is also called a porous layer.
Listed as features of the swelling type ink absorbing layer are achievement of high glossiness, absorption of a large amount of ink due to use of swellable polymers within the swellable range, and low cost production. On the other hand, along with in an increase in recent ink-jet recording rate, various problems occur in terms of image quality.
On the other hand, listed as features of the void type ink absorbing layer are high ink absorption rate, minimized non-uniformity during printing, apparent surface dryness immediately after printing, and simultaneous compatibility between the water resistance and the ink absorption rate.
At present, both are employed depending on selection by operators. However, from the viewpoint of image quality, it is preferable to have the void type ink absorbing layer due to its higher ink absorption and faster ink drying.
Incidentally, in recent years, it has been attempted to prepare printed matter and printing proofs employing ink-jet recording. In such cases, it is necessary to not only achieve approaching the tone of finished images to that of printed matter but also to match the tone of the white background to the specified tone. In such cases, required as the tone of white backgrounds are various types depending on surface quality, personal taste, intended use, and localities. Employed as a means to adjust the white background to the desired tone are methods in which supports, as well as ink absorbing layers, are slightly tinted with colorants such as dyes or pigments.
When the white background tone of the surface of the ink absorbing layer is varied to several levels, the difference in coloring of the support is not always identically obtained in the ink absorbing layer, and depends on the type of the ink absorbing layer. As a result, in the support coloring method, complicated white background control is required.
On the other hand, it is relatively easy to adjust the tone of the white background on the ink receptive layer. Accordingly, that is preferable since it is possible to obtain ink-jet recording sheets having various white backgrounds while employing a single support. The white background of the ink receptive layer is generally adjusted by incorporating a very small amount of dyes and pigments.
Incidentally, as noted above, in recent years, along with realization of high speed ink-jet printing, recording sheets comprising a porous ink receptive layer have been e mainly mployed. In such cases, as the porous ink receptive layer is employed, it has been found that problems tend to occur in which the tone varies during storage.
Namely, when the ink absorbing layer is porous, coloring components in the white background portion are subjected to greater change compared to the cases in which the ink receptive layer is comprised of a swelling layer. As a result, differing white background tone occasionally results during storage. Particularly, when a plurality of sheets is stored while stacked, it often occurs that edge portions are subjected to discoloration or the uppermost sheet is subjected to discoloration during storage. Considered as one of the causes is the following. Since the ink absorbing layer is comprised of a porous layer, it tends to react with oxidizing materials such as oxygen, whereby colorants such as dyes are relatively easily decomposed.
Claim 1 of Japanese Patent Publication No. 3012580 describes an ink-jet recording sheet which comprises a support having thereon an ink receptive layer and in which L*, a*, and b*, representing tone of the white background portion, which are measured based on JIS Z 8722 and specified by JIS Z 8730 are L*≧87, a*=−2 to 2, and b*=−3 to 3, respectively.
JIS Z 8722 is a Japanese Industrial Standard, and conforms with ISO/DIS 7724; Paints and varnishes, published in 1997.
JIS Z 8730 is a Japanese Industrial Standard, and conforms with the method for specification provided in clause 4 of Publication CIE No. 15.2(1986) COLORIMETRY, SECOND EDITION revised in 1986 by Commission International de l'Eclairage—CIE in short.
A set of coordinates (L*, a*, b*) can be obtained using a L*a*b* color system, which is termed the CIE1976(L*a*b*) and is abbreviated as CIELAB.
Further, claim 2 of the aforesaid patent publication also describes an ink-jet recording sheet which comprises a support having thereon an ink receptive layer, in which the whiteness of the white background portion of the support has L*≧87, a*=−2 to 2, and b*=−5 to 0, respectively.
According to claim 1 of the aforesaid patent publication, even in the case of employing a support having any tone, when L*, a*, and b* of the ink receptive layer surface are adjusted to the aforesaid range, the preferred white background is obtained.
Further, according to claim 2 of the aforesaid patent publication, when the support, having L*, a*, and b* in the aforesaid range, is used, preferred whiteness of the ink receptive layer surface is achieved even though the ink receptive layer, having an optional degree of coloration is coated and the ink receptive layer surface exhibits white background characteristics having optional values of L*, a*, and b*.